The prolactin receptor (PRLR) is a transmembrane receptor that interacts with prolactin (PRL), a peptide hormone. PRLR contains a single transmembrane domain and is homologous to receptors for cytokines, such as IL2, IL3, IL4, IL6, IL7, erythropoietin, and GM-CSF. PRLR is present in mammary glands, ovaries, pituitary glands, heart, lung, thymus, spleen, liver, pancreas, kidney, adrenal gland, uterus, skeletal muscle, skin, and areas of the central nervous system (Mancini, et al., Endocrinol Metab Clin North Am, 2008, 37(1):67-99). Upon activation by prolactin, PRLR dimerizes, resulting in activation of Janus kinase 2, a tyrosine kinase that initiates the JAK-STAT pathway and also results in the activation of mitogen-activated protein kinases and Src kinase. Growth hormone also binds to PRLR and activates the receptor.
PRLR is involved in multiple biological functions, including cell growth, differentiation, development, lactation and reproduction. The human PRLR cDNA was originally isolated from hepatoma and breast cancer libraries (Boutin et al., Molec. Endocr. 3: 1455-1461, 1989). The nucleotide sequence predicted a mature protein of 598 amino acids with a much longer cytoplasmic domain than the rat liver PRLR. The PRLR gene resides in the same chromosomal region as the growth hormone receptor gene, which has been mapped to 5p13-p14 (Arden, et al., Cytogenet. Cell Gene 53: 161-165, 1990).
The genomic organization of the human PRLR gene has been determined (Hu, Z.-Z. et al., J. Clin. Endocr. Metab. 84: 1153-1156, 1999). The 5-prime-untranslated region of the PRLR gene contains 2 alternative first exons: E13, the human counterpart of the rat and mouse E13, and a novel human type of alternative first exon termed E1N. The 5-prime-untranslated region also contains a common noncoding exon 2 and part of exon 3, which contains the translation initiation codon. The E13 and E1N exons are within 800 base pairs of each other. These 2 exons are expressed in human breast tissue, breast cancer cells, gonads, and liver. Overall, the transcript containing E13 is prevalent in most tissues. The PRLR gene product is encoded by exons 3-10, of which exon 10 encodes most of the intracellular domain. The E13 and E1N exons are transcribed from alternative promoters PIII and PN, respectively. The PIII promoter contains Sp1 and C/EBP elements that are identical to those in the rodent promoter and is 81% similar to the region −480/−106 in the rat and mouse. The PN promoter contains putative binding sites for ETS family proteins and a half-site for nuclear receptors.
PRLR exists in a number of different isoforms that differ in the length of their cytoplasmic domains. Four PRLR mRNA isoforms (L, I, S1a, and S1b) have been found in human subcutaneous abdominal adipose tissue and breast adipose tissue (Ling, C. et al., J. Clin. Endocr. Metab. 88: 1804-1808, 2003). In addition, expression of both L-PRLR and I-PRLR has been detected in human subcutaneous abdominal adipose tissue and breast adipose tissue using immunoblot analysis. Recent reports have suggested that PRLR is expressed and activated in human breast cancer and prostate cancer tissues (Li et al., Cancer Res., 64:4774-4782, 2004; Gill et al., J Clin Pathol., 54:956-960, 2001; Touraine et al., J Clin Endocrinol Metab., 83:667-674, 1998). It was reported that Stat5 activation and PRLR expression is associated with high histological grade in 54% of prostate cancer specimens (Li et al., supra). Other reports have suggested that primary breast cancer specimens are responsive to PRL in colony formation assays and that plasma PRL concentrations correlate with breast cancer risk (Tworoger et al., Cancer Res., 64:6814-6819, 2004; Tworoger et al., Cancer Res., 66:2476-2482, 2006). Another report indicated that PRL transgenic mice develop malignant mammary carcinomas or prostate hyperplasia (Wennbo et al., J Clin Invest., 100:2744-2751, 1997; Wennbo et al., Endocrinology, 138:4410-4415, 1997).
A PRLR monoclonal antibody has been shown to diminish the incidence of mammary tumors in mice (Sissom et al., Am. J. Pathol. 133:589-595, 1988). In addition, a PRL antagonist (S179D mutant PRL) has been shown to inhibit proliferation of a human prostate carcinoma cell line, DU-145, in vitro and DU-145 induced tumors in vivo (Xu et al., Cancer Res., 61:6098-6104, 2001).
Accordingly, there remains a need for PRLR binding proteins that can be used for therapeutic purposes for treating cancer.